1. Field of the Invention
The present invention relates to a printer belt drive control circuit and in particular, to a color electro-photographic printer belt drive control circuit.
2. Description of the Related Art
Explanation will be given on a conventional printer belt drive control circuit with reference to the attached drawings.
FIG. 6 is a schematic side view of a printer belt drive control scheme as a first conventional example (as disclosed in Japanese Patent Publication 03-133670). The printer belt drive control scheme shown in FIG. 6 includes rotary drums 225a and 225b, a photosensitive belt 221 placed over the rotary drums 225a and 225b, an encoder 223 arranged on a pulley for controlling the feed speed of the photosensitive belt 221; rotary drums 226a and 226b, a paper feed belt 222 placed over the rotary drums 226a and 226b, and an encoder 224 arranged on a pulley for controlling the feed speed of the paper feed belt 222.
In such a scheme, one color printing process includes a cycle of exposure, development, and transfer repeated for the number of colors of the developer and it is very difficult to overlap the color images. Especially when an eccentricity is present in the rotary drums 225a and 225b feeding the photosensitive belt 221 or in the rotary drums 226 and 226b feeding the paper feed belt 222 or in the pulleys of the encoders 223 and 224, the respective color images are obtained out of the ideal image positions, disabling the correct overlap of the color images.
If it is assumed that an image data write is started by a laser beam oscillation from a laser beam oscillator (not depicted) upon reception of a detection signal of the top of a paper (not depicted) from an image top detection sensor (not depicted), and if it is assumed that the time t1 , required for the paper top to travel from an image top detection position B to a transfer position A is equal to the time t2 required for the write tip of the photosensitive belt 221 to travel from a write position C to the transfer position A, then an image exposed and developed is transferred to the paper, starting from the paper top.
t1=x1/v1, t2=x2/v2
therefore,
xe2x80x83x1/v1=x2/v2
wherein v1 is a feed speed of the photosensitive belt 221 detected by the encoder 223; V2 is a feed speed of the paper feed belt 222 detected by the encoder 224; x1 is a feed distance of the photosensitive belt 221 from the write position C to the transfer position A; and x2 is a feed distance of the paper feed belt 222 from the image top detection position B to the transfer position A.
If the feed speed v1 of the photosensitive belt 221 is different from the feed speed v2 of the paper feed belt 222, then a slip is caused between the photosensitive belt 221 and the paper at the transfer position A. Accordingly, it is assumed that
v1=v2.
Accordingly, if the feed speed v1 of the photosensitive belt 221 and the feed speed v2 of the paper feed belt 222 are maintained at a constant value by the encoders 223 and 224 and if the feed distance x1 of the photosensitive belt 221 and the feed distance x2 of the paper feed belt 222 are maintained at a constant value, the color images at the respective cycles of exposure, development, and transfer are completely matched with one another.
However, if an eccentricity is present in one of the rotary drums 225a, 225b, 226a, and 226b (as shown by a dotted-broken line in FIG. 6), the feed distance x1 of the photosensitive belt 221 and the feed distance x2 of the paper feed belt 222 vary for different cycles of exposure, development, and transfer of respective color images, and the time t1 does not coincide with the time t2, causing mis-overlap of different color images.
The same happens if an eccentricity is present in the pulley of the encoder 223 or 224.
FIG. 7 is circuit diagram and FIG. 8 is a servo block diagram of a second conventional example.
A belt motor 6 is provided with only a motor encoder 7 and a belt is controlled by feed back of the motor speed information. There is no consideration on the allowance or rigid components of the gears in a drive transmission mechanism. To obtain a constant rotation of the belt via a drive roller, it has been difficult to constitute a stable belt control system.
It is therefore an object of the present invention to provide a belt drive control circuit enabling to rotate a belt at a stable constant speed.
The printer belt drive control circuit according to the present invention includes: an encoder attached to a drive roller for driving a belt to be controlled, and a servo circuit for controlling a rotation speed of a motor for driving the drive roller to be constant according to an output signal from the encoder.
According to another aspect of the present invention, the printer belt drive control circuit includes: a first encoder attached to a drive roller for driving a belt to be controlled, a second encoder attached to a main shaft of a motor for driving the drive roller, and a servo circuit for controlling a rotation speed of a motor driving the drive roller to be constant according to an output signal from the first encoder and an output signal from the second encoder.
According to still another aspect of the present invention, the printer belt drive control circuit includes: an encoder count circuit (4) for counting output signals from a drive encoder (11) attached to a drive motor (10) and from a motor encoder attached to a belt motor (6), a servo controller which under control of a CPU (1) and outputs a rotation speed information of the belt motor (6) according to tan output signal from the encoder count circuit (4), and a motor drive signal generation circuit for converting an output information from the servo controller (2) into a form appropriate for an H-bridge driver circuit (5) which directly drives the belt motor (6).
According to yet another aspect of the present invention, the servo controller includes: an internal ROM contains a program of a control algorithm used for driving a belt (12) and computation equations for control by a software servo method, and an internal register containing servo constants and servo parameters used for a servo computation, which are read out, when necessary, to be substituted in a computation equation.
According to still yet another aspect of the present invention, the belt drive control circuit (8) is controlled in a closed loop.
According to further another aspect if the present invention, the belt drive control circuit (8) is controlled in a closed loop using software servo control.